A three-phase motor (such as a permanent magnet synchronous motor and induction motor) can be used in many applications to provide power assistance to mechanical systems. It is known to control the phase windings in a three-phase motor using pulse width modulated signals. The pulse width modulated signals are applied to an inverter or a series of switching devices that connect the phase windings of the motor to either a positive or negative terminal of a direct current (DC) power supply, such as a battery. This inverter consists of three switches in an upper bank and three switches in a lower bank.
To adequately control the motor, it is necessary to measure the current flowing through each phase winding. The current measured in each phase is provided to a controller that generates the pulse width modulated signals. To measure currents, it is known to use a resistor in series with each phase winding. The voltage drop across each resistor is measured to determine the current flowing for each phase. This type of system has the disadvantage of requiring three current sensors, one for each of the three phase windings, and three measurement circuits.
It is also known to eliminate at least one of the current sensors and calculate the current through the phase without a sensor based on the measurements of the other two phases. In most cases the sum of all currents flowing through all phases of the motor are zero, so by knowing the current flowing through two phases a determination of the current flowing through the phase without a sensor may be made (with the exception of star-connected motors with a grounded star point or a motor with a fault).
It is also known to use a single resistor to perform the function of measuring the current through each phase of the three-phase motor. The single resistor is located external to the motor windings on the DC link between the DC power supply and the inverter or series of switching devices. Depending on the states of the switches and period of operation, the currents through the phases may be measured or calculated by the single resistor.
It is also known to close either the upper bank of switching devices or the lower bank of switching devices, thereby connecting all phases of the windings of the motor together when damping or braking is required. Braking is used to damp unwanted mechanical system oscillations and is also used to reduce the speed of the mechanical system. For example, mechanical resonances or disturbances may feed back into the motor from the mechanical system. Not only are unstable mechanical resonances undesired by themselves, these resonances can generating an electromotive force (EMF) that can result in excess current in the motor. In damping mode, the closed switches allow current to circulate within the motor windings to act as a brake to oppose any motion induced by the mechanical resonances, thereby helping to restore stability. However, the internal circulation of currents within the motor windings in damping mode cannot be measured by a single external resistor since the resistor is not in a current carrying loop. Therefore, the currents within the motor windings cannot be measured, and therefore cannot be modified or controlled.
It is also known to modulate the three phase inverter switches to force the current in the motor to return to the battery for increased braking effort. In this case, the battery current flowing through the single resistor is in the opposite direction compared to normal motor operation. Consequently, the voltage developed across this resistor is negative and cannot be read by the controller unless the controller is designed for this case.
Therefore, a need exists for an improved technique to monitor current in a three-phase device, during damping/braking mode, using a single current sensor. It would also be desirable to provide this technique with no additional hardware requirements, thereby improving reliability and saving cost.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the broad scope of the invention as defined by the appended claims.